Rename GWcs to GSkyProjection.

WCS should be reserved for spherical world coordinate system projections, and a more general term should be used for sky projections.
Replace GWcs by GSkyProjection.

After renaming GWcs to GSkyProjection the code has been adapted to the new naming convention.
Rename GWcsHPX to GHealpix.
Replace GWcsHPX by GHealpix.

The Healpix pixelisation is not a World Coordinate System projection, and to make this clear the name has been changed from GWcsHPX to GHealpix. The code has been adapted to this new naming convention.
Rename GWcslib to GWcs.
Replace GWcslib by GWcs.

The GWcslib class has been renamed to GWcs for consistency with the naming convention for GammaLib base classes. All WCS implementations derive now from GWcs. The code has been adapted to this new naming convention.
Generalise GSkyPixel to handle both 1D and 2D sky map pixels.

GSkyPixel is now able to handle both 1D and 2D sky map pixels. 1D sky map pixels behave like int or double types, and conversion from and to these types is supported. 2D sky map pixels store the X and Y component of the pixel. Methods are provided to test whether a pixel is 1D or 2D.
Implement generic GSkyPixel class and rename pixel/index/direction conversion methods consistently.

The GSkyPixel class now handles 1D and 2D sky indices. This made the distinctive 1D and 2D methods in GSkyProjection and the derived classes obsolete. Now, all projection classes take GSkyPixel as argument.

The GSkymap class now uses the universal GSkyPixel class. Methods have been renamed consistently. An integer index is now consistently called "sky map index" and the GSkyPixel object is named a "sky map pixel". Methods operating on indices contain now "inx", method operating on sky map pixels "pix".
Add sky map example code.
Ignore example script executables.
Update after sky module review